APPLICANT'S ABSTRACT: Sharing syringes is recognized as the primary risk for HIV transmission among injection drug users (IDUs). However, little is known about a number of injection-mediated practices included under the term "indirect sharing" (including such practices as backloading and the sharing of rinse water or filters for drug solutions) despite increasing evidence of their implication in transmission of HIV. This study will integrate ethnographic and survey research on the nature and prevalence of these practices with laboratory studies of their effects on viral activity and viability. The specific aims of this study are to describe the nature of these practices, develop a typology of social-network and physical injection settings, and establish the associations of these settings with indirect sharing practices. The typology will incorporate both the specific settings in which injection occurs and community-level contexts of contrasting paraphernalia availability and background seroprevalence that characterize the two cities --New York and Denver -- within which methodologically parallel studies will be conducted. Other aims are to assess the potential risk of these injection practices for HIV transmission, and estimate their prevalence within the IDU populations of the two cities. Ethnographic methods will document the practices by which IDUs obtain, prepare, and inject drugs, with particular attention to injection paraphernalia and techniques for its use, and develop a typology of contexts within which these practices occur.Laboratory simulation methods will then assess the potential presence and viability of viral materials associated with each of these practices as they have been observed by the ethnographic studies. A survey of IDUs (200 in each city) will estimate the prevalence of the practices identified by laboratory simulation as carrying a high risk of HIV transmission. This project will advance our knowledge of the specific processes of HIV transmission and their relative frequency. It will contribute to prevention by evaluating the relative risk attributable to each, and the efficacy of common disinfection methods in preventing the spread of HIV infection.